1. Field of the Invention
The present invention relates generally to planar antennas and, more particularly to a small planar antenna which can be suitably and unitarily formed with mobile communication equipment or the like.
2. Description of the Prior Art
Simplified and miniaturized planar antennas of low profile are generally utilized as an antenna system in the fields of satellite communication and mobile communication.
A microstrip antenna, which is one of the most typical planar antennas, generally utilizes circular or rectangular radiation elements.
The dimension of the radiation elements of these configurations is uniquely determined in response to the frequency used.
In the satellite communication and mobile communication fields, it is a fundamental request that the antennas are miniaturized. Therefore, when the planar antenna is unitarily formed with a high frequency circuit or when the whole communication equipment including the antenna system is unitarily formed as one unit, the rectangular radiation element having an excellent space factor is well matched with the high frequency circuit, the communication equipment or the like as compared with the circular radiation element.
Further, in the above-mentioned communication field, circularly-polarized waves are frequently utilized. To this end, in the conventional planar antennas, as shown in FIGS. 1 to 3, rectangular radiation elements are deformed in a predetermined deformation manner such as cut-away, extension, increase of width or the like in order to effect degeneration and separation. Also, a single feed point is disposed at a proper position on these radiation elements as shown in FIGS. 1 through 3.
As shown in FIG. 1 of the accompanying drawings, a pair of recesses 1C are formed on both ends of one diagonal line of a rectangular radiation element 1 and a single feed point 2 is disposed on the radiation element 1 at the position properly offset from the center of the radiation element 1 parallel to one side, whereby the radiation element 1 is driven in two modes perpendicular to each other along the two diagonal lines as shown by arrows 3a and 3b in FIG. 1.
These two modes are considered as synthesized modes of TM.sub.10 and TM.sub.01. However, if the recesses 1C are not formed on the radiation element 1 as shown by broken lines in FIG. 1, then two modes 3a, 3b are resonated at the same frequency and cannot be discriminated from each other from the outside, which state will be referred to as degeneration.
If the pair of recesses 1c are formed and perturbed as shown in FIG. 1, then the portions of the recesses 1c act as a strong electric field area for one mode 3a and also act as a strong magnetic field area for the other mode 3b so that the amounts in which resonant frequencies of the respective modes 3a, 3b are displaced by the existence of the recesses 1c become different. As a consequence, the two modes 3a and 3b are resonated at different frequencies and released (separated) from the degenerated state. Therefore, the two modes can be discriminated from each other from the outside.
As described above, the planar antenna having the rectangular radiation element shown in FIG. 1 can generate a circularly-polarized wave by the single feed point 2 by applying the perturbation to the recesses 1c so as to make the exciting phase difference become 90 degrees.
Further, in a rectangular radiation element 1S shown in FIG. 2, the recesses 1c of FIG. 1 are replaced with stubs 1b and a circularly-polarized wave can be generated by the single feed point 2 similarly as described above.
Furthermore, in a rectangular radiation element 1W of FIG. 3, a width l thereof is increased by a proper amount (2.multidot..DELTA.l) and a single feed point 2 is disposed on one diagonal line of the radiation element 1W at the position properly offset from the center of the radiation element 1W, whereby the radiation element 1W is driven in two orthogonal modes parallel to the respective sides as shown by arrows 3a and 3b.
The radiation element 1W shown in FIG. 3 is perturbed at the extended width portion 1sp so as to provide an exciting phase difference of 90 degrees, thereby making it possible to generate a circularly-polarized wave by the single feed point 2.
In any of the above-mentioned three examples, a relation is established between an area S of an original rectangular radiation element and an area .DELTA.S of a degenerated or separated portion (recess, stub, widened portion) as expressed by the following equation (1): EQU .DELTA.S/S=1/2.multidot.Qo (1)
where Qo is the no-load Q of the planar antenna.
When the planar antenna itself is miniaturized, such a method is known to reduce the dimension of the radiation element by changing a ratio between sides so that a length thereof in the direction perpendicular to the exciting direction 3 defined by the position of the feed point 2 is reduced, that is, the rectangular radiation element 1 shown in FIG. 4A is reduced to a radiation element 1m shown in FIG. 4B.
Further, according to the following known method, the dimension of the radiation element is reduced by short-circuiting the radiation element 1 to a ground conductor 5 at a zero potential line 4 passing the center of the original radiation element 1 and which is perpendicular to the excitation direction 3 as if the rectangular radiation element 1 shown in FIG. 5A is reduced to a radiation element 1h shown in FIGS. 5B and 5C.
However, in the conventional miniaturized planar antennas shown in FIGS. 4 and 5, the lengths of the radiation element in the excitation direction and lengths perpendicular to the excitation directions are very different from each other, that is, a so-called isotropic property of the radiation element is deteriorated. As a consequence, independent orthogonal modes cannot be realized at substantially equal resonance frequencies and therefore circularly-polarized waves cannot be generated. For this reason, the conventional planar antenna cannot be utilized in fields of circularly-polarized wave communication such as a mobile communication or the like.